1. Field of the Invention
The present invention relates to an optical displacement measuring apparatus for optically measuring a state, an amount, or a speed of displacement of an object. In this description, the word "a state, an amount or a speed of a displacement" is intended not only to mean a moving state, a moving amount and a moving speed of a moving object, but also to mean a changing state, a changing amount and a changing speed of an object which is being deformed or is being changed in its shape or figure.
2. Description of Related Art
There has been conventionally proposed a speckle image technique for optically measuring a displacement amount of a moving object.
Japanese Unexamined Patent Application Publication No. 59-212773 discloses an optical velocity measuring apparatus applying the speckle image technique. In the optical velocity measuring apparatus, a moving object is illuminated with laser beam at two timings (first and second timings), so that a speckle pattern is formed at each of the two timings. (The speckle pattern formed at the first timing will be referred to as a "first speckle pattern", and the speckle pattern formed at the second timing will be referred to as a "second speckle pattern", hereinafter.) The first and second speckle patterns have such a relationship that the speckles constituting the second speckle patterns are distributed in the same manner as the speckles constituting the first speckle pattern, but the speckles of the second speckle pattern are shifted as a whole from the speckles of the first speckle pattern, since the object is moved between the two timings. More specifically to say, each of the speckles of the second speckle pattern is shifted from a corresponding one of the speckles of the first speckle pattern, with a shift amount being determined based on the displacement amount of the object attained between the two timings.
In the optical velocity measuring apparatus, each of the first and second speckle patterns is detected by a plurality of optical detecting elements. A correlation (shift amount) between the first and second speckle patterns is calculated, while shifting the detected results for the first speckle pattern relative to those for the second speckle pattern. Based on the thus obtained correlation (shift amount) between the first and second speckle patterns, the displacement amount of the object attained between the two timings and the velocity of the object are calculated.
In the above-described conventional optical velocity measuring apparatus, however, a lot of time is required to detect each of the first and second speckle patterns with the plural optical detecting elements and to calculate the correlation between the first and second speckle patterns. In other words, the velocity measuring operation of the conventional apparatus is complicated and therefore requires a lot of time. Accordingly, the conventional optical velocity measuring apparatus fails to realize a real-time velocity detecting operation. In addition, since the velocity measuring operation requires a lot of time, the conventional velocity measuring apparatus fails to detect changes in the velocity of the object with a short time interval.